The invention relates generally to filter-fluorescer combinations for measuring short bursts of high-fluence x-rays, and more particularly, it relates to measuring such bursts in energies above 120 keV, where no practical material absorption edges exist for conventional filter-fluorescer techniques.
To determine the fluence in different energy bands below 120 keV emitted by a high-fluence x-ray source, the prior art uses a pre-filter, a fluorescer, and a post-filter, all with properly selected absorption edges. The responses of this combination are shown in graphs a-d of FIG. 1. With the pre-filter absorption edge E.sub.PRF determining the upperbound of the energy band of interest, and the fluorescer E.sub.F and post-filter E.sub.POF absorption edges coinciding with the lower bound of the energy band, the result is a joint response function with a sharply defined high transmission in the energy band E.sub.F &lt;E&lt;E.sub.PRF, a sharp transmission fall-off for energies E&lt;E.sub.F and markedly attenuated transmission for energies E&lt;E.sub.PRF. As used herein, an absorption edge is the energy corresponding to an absorption discontinuity in the intensity of an x-ray absorption spectrum, which gives the appearance of a sharp edge in such a spectrum. Also, an energy band is a zone between two different energies in the x-ray spectrum. Combinations to achieve a sharply defined type of high transmission are described in the papers "Filter Fluorescer Experiment on the Argus Laser" (preprint UCRL-81471) published Oct. 10, 1978, by H. N. Kornblum et al; and "A Ten Channel Filter-Fluorescer Spectrometer" (preprint UCRL-81477) published Oct. 31, 1978, by B. L. Pruett et al.
In a paper entitled "Filtered Detector Arrays for Single Pulsed Photon Measurements Above 100 keV" (preprint UCRL-80314) published Nov. 9, 1977, by K. G. Tirsell and H. C. Catron, the authors show that the spectrometer range may be extended to x-ray energies around 250 keV by replacing the filter-fluorescer combination with a simple filter-detector combination; i.e., a combination including only filters and detectors. The reason for this replacement in prior art is that above 120 keV there are no practical absorption edges available for conventional filter-fluorescer techniques. As shown in FIG. 2, the result with the simple filter-detector combination is a response function having an energy transmission band of 200 to 5000 keV width. This bandwidth, however, is too broad for energy discrimination purposes. Furthermore, one or more undesirable windows (such as W1 and W2 of FIG. 2) appear at lower bands corresponding to lower absorption edges of the filter material. These windows introduce a large error in the detector signal when the spectrum being measured is a rapidly decreasing function of the x-ray energy. This limitation is particularly troublesome when measuring x-rays from a gold source, since the windows occur in the region where the gold K-lines are present.
These problems related to the filter-fluorescer technique, establish the need for a well-defined response function when measuring high-fluence x-ray energies above 120 keV.
It is, accordingly, a general object of the invention to measure high-fluence x-rays above 120 keV energy, where there are no practical absorption edges available, for conventional filter-fluorescer techniques.
Another object of the invention is to improve the channel response function in measuring high-fluence x-rays above 120 keV by narrowing the energy transmission band to a width suitable for energy discrimination purposes.
Another object is to eliminate undesirable response windows in measuring high-fluence x-rays above 120 keV and to ensure that these windows vanish in the region where the gold K-lines appear.
Other objects, advantages, and novel features of the invention will be apparent to those of ordinary skill in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.